Hyaluronic acid ("HA") is a naturally occurring linear polysaccharide composed of alternating disaccharide units of N-acetyl-D-glucosamine and D-glucuronic acid joined by alternating .beta.1.fwdarw.3 glucuronidic and .beta.1.fwdarw.4 glucosaminidic bonds, so that the repeating unit is (1.fwdarw.4)-.beta.-D-GlcA-(1.fwdarw.3)-.beta.-D-GlcNAc. The disaccharide unit of hyaluronic acid, or a salt thereof, may be represented in the following way: ##STR1## wherein Ac represents acetate and R represents hydrogen (in the case of the acid) or the cation of a salt (in the case of a salt). Preferably, the cation is an alkali metal cation, most preferably sodium ion. Hereinafter, formula I above shall be referred to in the following way: ##STR2## wherein R is as defined above and B has the obvious meaning ascribed to it.
HA is widely distributed in animal tissues, present in high concentrations in synovial fluid and the vitreous body of the eye, and in connective tissues of rooster comb, umbilical cord, and dermis. The molecular weight of hyaluronic acid isolated from natural sources generally falls within the range of about 6.times.10.sup.4 to about 1.2.times.10.sup.7 daltons. Naturally occurring HA does not give a foreign body reaction when implanted or injected into a living body and it has excellent biocompatibility.
As used herein, the term "HA" means hyaluronic acid and any of its hyaluronate salts, including, but not limited to, sodium hyaluronate (the sodium salt), potassium hyaluronate, magnesium hyaluronate, and calcium hyaluronate.
HA, in chemically modified form, is useful as a surgical aid, to prevent adhesions or accretions of body tissues during the post-operation period. The modified HA composition (e.g., gel or film) is injected or inserted into the locus between the tissues that are to be kept separate to inhibit their mutual adhesion.
Chemically modified HA is also useful for controlled release drug delivery. Sparer, R. V. et al., 1983, Chapter 6, pp. 107-119, in T. J. Roseman et al., Controlled Release Delivery Systems, (Marcel Dekker, Inc., New York), describe the sustained release of chloramphenicol covalently attached to hyaluronic acid via ester linkage, either directly or in an ester complex including an alanine bridge as an intermediate linking group.
The literature describes two general approaches for chemically modifying HA to reduce its water solubility and diffusibility in vivo: (a) cross-linking HA by bifunctional chemical reagents and (b) coupling HA by monofunctional reagents.
Divinyl sulfone, bisepoxides, formaldehyde, and bishalides are bifunctional reagents which have been used to cross-link HA to produce highly swollen gels or virtually insoluble, plastic materials, depending upon the degree of cross-linking. Balazs, E. A. and Leshchiner, A., U.S. Pat. No. 4,582,865, describe the use of divinyl sulfone in an alkaline medium to cross-link HA. Balazs, E. A., Leshchiner, A., U.S. Pat. No. 4,713,448, describe a chemically modified HA preparation characterized by the presence of aldehyde cross-linking groups, such as formaldehyde, covalently bonded to the HA chains. Maelson, T. and Lindqvist, B. P., PCT Application WO-86-79A1, describe a method of preparing crosslinked gels of HA by reaction with a phosphorus-containing agent. De Belder, A. M. and Maelson, T., PCT Application WO-86 00912, describe a slowly-degradable gel, for preventing tissue adhesions following surgery, prepared by cross-linking a carboxyl-containing polysaccharide with a bi- or poly-functional epoxide.
There are other reactive bi- or polyfunctional agents, which have been proposed for preparing cross-linked gels of HA having reduced water solubility. For example, Balazs et al., U.S. Pat. No. 4,582,865, suggest that divinyl sulfone may be used to prepare cross-linked gels of HA having reduced water solubility. In addition, Balazs et al., U.K. Patent Application No. A4 20 560, suggest that agents such as formaldehyde, dimethylolurea, dimethylolthylene, ethylene oxide, polyaziridine, and polyisocyanate can be used to prepare cross-linked gels of HA having reduced water solubility.
Other approaches used to render HA compositions less water soluble by cross-linking the HA include modifying HA by attaching cysteine residues to the HA via amide bonds and then cross-linking the cysteine-modified HA by forming disulfide bonds between the attached cysteine residues. The cysteine-modified HA was itself water soluble and became water insoluble only upon cross-linking by oxidation to the disulfide form. Sparer, R. V. et al., 1983, chapter 6, pp. 107-119, in T. J. Roseman et al., Controlled Release Delivery Systems (Marcel Dekker, Inc., New York).
Coupling reactions have also been shown to alter the properties of HA. For example, extensive esterification of HA with monofunctional organic halides can produce water-insoluble films. Della Valle, F. and Romeo, A., European Patent Application 87308863.8.
Danishefsky, I. et al., Carbohydrate Res. 16: 199-205 (1971), describe modifying a mucopolysaccharide by converting the carboxyl groups of the mucopolysaccharide into substituted amides by reacting the mucopolysaccharide with an amino acid ester in the presence of 1-ethyl-3-(3-dimethyliaminopropyl) carbodiimide hydrochloride ("EDC") in an aqueous solution. The authors reacted glycine methyl ester with a variety of mucopolysaccharides, including HA. Daniskefsky et al. reported that the resulting products were water soluble. That is, they would rapidly dissolve in water or in an aqueous solution such as is encountered between body tissues.
Amidation reactions of HA and monofunctional amines catalyzed by carbodiimides have been shown to decrease water solubility. Hamilton et al., U.S. Pat. No. 4,937,270, describe a method for making a water-insoluble biocompatible gel by activating HA with a carbodiimide then reacting the activated HA with a nucleophile (e.g., an amine). In the presence of a primary amine as nucleophile, the O-acylisourea formation is followed by a nucleophilic attack, forming an amide linkage between the amine and the carboxylic acid.
Others have shown that when a mixture of HA and other polyanionic polysaccharides react with a carbodiimide, a water-insoluble gel is formed. Burns et al., U.S. Pat. No. 5,017,229, describe a method of making a water-insoluble biocompatible gel by reacting HA, another polyanionic polysaccharide and a carbodiimide.